1. Field of the Invention
The present invention relates to a radio frequency transceiver, and more particularly, to a radio frequency transceiver used in a wireless digital telephone for transmitting and receiving radio frequency signals.
2. Description of the Prior Art
Many wireless digital telephones use dual conversion transceivers to transmit and receive RF (radio frequency) signals. Most dual conversion transceivers require two local oscillators to generate two different local oscillator signals for modulating and demodulating signals. Two prior art dual conversion transceivers are described in FIGS. 1 and 2.
Please refer to FIG. 1. FIG. 1 shows a system block diagram of a prior art radio frequency transceiver 10 which comprises two local oscillators 140 and 150. The transceiver 10 comprises an antenna 101 for transmitting and receiving RF signals, a duplexer 102 for filtering out noises and interference signals contained in the received RF signal and for matching impedances between the antenna 101 and the follow-on circuits, a LNA (low noise amplifier) 111 for amplifying the filtered RF signal, a front end filter 112 for filtering out noises contained in the amplified RF signal, a mixer 113 for mixing the filtered RF signal from the front end filter 112 with a first local oscillator signal from the first local oscillator 140 to generate an IF (intermediate frequency) signal, an IF filter 114 for filtering out spurious signals contained in the IF signal, and a demodulator 120 for demodulating the filtered IF signal from the IF filter 114 by using second local oscillator signal from the second local oscillator 150 into baseband signals which will be outputted over port 124 for internal processing.
The transceiver 10 further comprises a modulator 10 for modulating baseband signals inputted from ports 191 and 192 into an IF signal by using the second local oscillator signal from the second local oscillator 150, an IF filter 170 for filtering out spurious signals contained in the IF signal, a mixer 171 for modulating the filtered IF signal from the IF filter 170 into a RF signal by using the first local oscillator signals from the first local oscillator 140, a RF filter 172 for filtering out spurious signals contained in the RF signal from the mixer 171, and a power amplifier 173 for amplifying the filtered RF signal. The amplified RF signal are passed to the duplexer 102 for filtering out high order harmonic frequencies caused by nonlinear distortion of the power amplifier 173 and then transmitted by the antenna 101.
The demodulator 120 is a conventional quadrature demodulator and the modulator 160 is a conventional I/Q modulator. The modulator 160 includes a 90 degree phase shifter 163 for shifting the phase of the second oscillator signal from the second oscillator 150, an I mixer 161 for mixing the I baseband signal inputted from port 191 with the output of the phase shifter 163, a Q mixer 162 for mixing the Q baseband signal inputted from port 192 with the second oscillator signal, and an adder 164 for adding the outputs of the I mixer 161 and Q mixer 162 to generate the IF signal.
The first local oscillator 140 used for generating the first oscillator signal comprises a VCO (voltage controlled oscillator) 141 and a PLL (phase locked loop) 142. The PLL 142 is used for frequency stabilization and for input/output frequency switching according to control signals from port 144 when transmitting or receiving RF signals. The second local oscillator 150 used for generating the second local oscillator signal comprises a VCO 151 and a PLL 152. The PLL 152 is used to stabilize frequency of the second local oscillator 150 and to provide input and output frequency switching function according to control signals inputted from port 154.
A crystal oscillator (TCXO) 155 is used as a stable signal source for supplying reference signals to PLLs 142 and 152. Port 144 of the PLL 142 and port 154 of the PLL 152 are connected to a control unit (not shown) which is used to control signal transmitting and receiving of the transceiver 10.
The transceiver 10 further comprises a power amplifier control circuit 180 having a port 193 connected to the control unit for turning on the power amplifier 173 when the transceiver 10 is transmitting signals.
Please refer to FIG. 2. FIG. 2 shows a system block diagram of another prior art radio frequency transceiver 20 which comprises two local oscillators 240 and 250. The transceiver 20 comprises an antenna 201 for transmitting and receiving RF signals, a front end circuit 202 for filtering out noises and various interference signals from the received RF signal and for matching impedance between the antenna 201 and the follow-on circuits. The RF signal received by the antenna 201 will be passed though a switch 203 and the front end circuit 202 to a low noise amplifier 211 for signal amplification, and then inputted to a front end filter 212 for filtering out noises contained in the received RF signal. A mixer 213 is used for mixing the RF signal from the front end filter 212 with first local oscillator signal from the first local oscillator 240 to convert the RF signal into an IF signal. The IF signal will pass through an IF filter 214 for filtering out spurious signals generated by the mixer 213 in the signal mixing process. And a demodulator 220 will demodulate the filtered IF signal from the IF filter 214 by using second oscillator signal from the second local oscillator 250 into baseband signals which will be outputted over port 224 for further internal processing.
The transceiver 20 further comprises a modulator 260 for modulating baseband signals to be transmitted into an IF signal by using the second oscillator signal from the second local oscillator 250, a frequency multiplier 270 for converting the IF signal into a RF signal by using the first local oscillator signal from the first local oscillator 240, and a power amplifier 283 for amplify the RF signal. The RF signal generated by the frequency multiplier 270 require no filtering because the frequency multiplier 270 is an active feed back loop which can stably and accurately performs the frequency conversion work. The amplified RF signal outputted from the power amplifier 283 will be passed to the switch 203 and then transmitted by the antenna 201.
The demodulator 220 is a conventional quadrature demodulator. The modulator 260 is the same as the modulator 160 shown in FIG. 1. And the first and second local oscillators 240 and 250 are identical to the two local oscillators 140 and 150 shown in FIG. 1. A crystal oscillator (TCXO) 255 is used as a stable signal source for supplying reference signals to PLLs 242 and 252, and port 244 of the PLL 242 and port 254 of the PLL 252 are connected to a control unit (not shown) which is used to control signal transmitting and receiving of the transceiver 20.
The transceiver 20 further comprises a power amplifier control circuit 280 which includes a port 293 connected to the control unit for turning on the power amplifier 283 when the transceiver 20 is transmitting signals. The switch 203 also includes a port 235 connected to the control unit for controlling input and output of RF signals to and from the antenna 201.
The difference between the transceiver 10 and the transceiver 20 is that the frequency multiplier 270 of the transceiver 20 is an active feedback frequency multiplier which is very similar to a frequency synthesizer and is used for changing frequencies. The frequency multiplier 270 comprises a mixer 272 for mixing the first oscillator signal from the first oscillator 240 with the RF signal outputted from the frequency multiplier 270, a phase comparator 274 for detecting the phase difference between the output of the mixer 272 and the IF signal inputted from the modulator 260, a loop filter 276 for filtering out unwanted signals contained in the output of the phase comparator 274, and a voltage controlled oscillator 278 for generating the RF signal according to the voltage outputted from the loop filter 276. The frequency multiplier 270 is a close loop which functions as a mixer and a filter. Comparing the two transceivers 10 and 20, the use of the frequency multiplier 270 in transceiver 20 can save the high frequency duplexer 102 and the RF filter 172 which are quite expensive.
From the above two transceivers 10 and 20, it can be seen that traditional dual conversion transceivers require at least two oscillators for signal conversions which makes them quite complex and expensive.